The present invention relates to a roots type fluid machine wherein a pair of parallel rotary shafts is rotatably supported by a housing and each rotary shaft has disposed thereon a rotor which has at least two lobe portions and two well portions so that the rotors engage with each other, and each rotor is located in a rotor chamber of the housing.
A roots compressor that serves as a roots type fluid machine has a housing and a pair of two-lobe or three-lobe rotors located in a rotor chamber of the housing. The rotors are located in the rotor chamber so as to have minimum clearance with the peripheral surface of the rotor chamber and also between the rotors. The two-lobe rotors engage with each other every 90 degrees of rotation of the rotors and the three-lobe rotors every 60 degrees of rotation of the rotors. There is an involute type rotor a part of which is formed by an involute curve. The involute type rotor is formed so that its lobe portion tapers toward its tooth tip. Therefore, the involute type rotor of the roots compressor has a small moment of inertia and, therefore, the roots compressor can be rotated at a high speed. In addition, a large volume of fluid can be trapped between the rotors and the peripheral surface of the rotor chamber, so that the displacement per rotation of the rotor is increased, thus offering an improved compression performance.
Since the tooth tip of the lobe portion of the involute type rotor is thin and a recess is formed in the well portion of the rotor for preventing interference with the lobe portion, the engaged rotors have formed between the lobe portion and the well portion thereof a space. The fluid or gas trapped in the space is compressed, expands and then released to the rotor chamber in accordance with rotation of the rotor. When the fluid is released to the rotor chamber, a large noise is generated.
Japanese Unexamined Patent Application Publication No. 9-264277 discloses a roots compressor or a roots type fluid machine having rotors which permit trapping of a large volume of fluid while preventing the abnormal noise. In the roots compressor of the cited reference, the lobe portion and the well portion of the rotor are formed in the shape of a circular arc and the other part of the rotor is formed by an involute curve. By so forming the rotor, trapping of a large volume of fluid is ensured and a space is prevented from being formed between the lobe portion and the well portion and, therefore, the noise generation is prevented.
In prior art roots compressors including the above-described roots compressor, a phase shift may occur when the rotor is tilted by a load received by the rotor during operation of the compressor, or a phase shift may be caused also during assembling of the compressor. In the roots compressor having such a phase shift, the lobe portion of one rotor and the well portion of the other rotor interfere with each other, which causes trouble such as noise. To eliminate the trouble, a large clearance needs be set between the rotors of the roots compressor in view of the above phase shift. If a large clearance is provided, however, leak of the fluid through the clearance will be increased thereby to reduce the performance of the roots compressor. For this reason, when the high performance of the roots compressor is desired, the clearance must be small, which makes it hard to avoid the interference between the rotors due to the phase shift. Therefore, it is desired to provide a roots compressor which is capable of preventing a trouble caused by interference between the rotors due to the phase shift.
The present invention is directed to a roots type fluid machine which prevents a trouble caused by the interference between the rotors due to the phase shift while ensuring trapping of a large volume of fluid.